Speaker efficiency vs sensitivity - Home Theater Forum and Systems - HomeTheaterShack.com

Old 06-15-10, 03:42 AM Thread Starter
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Ice10

Join Date: Apr 2007
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Speaker efficiency vs sensitivity

Hi Guys,

Not sure if this is the correct forum to be posting this. So I was having this discussion concerning efficiency and sensitivity and some of the guys were lumping the terms together as if they were exchangeable.

The one EE I discussed this with said both terms vary in the same way and there is a direct relationship between the two terms. He then went on to say that doubling the efficiency would net a 3 dB increase in sensitivity.

I don't agree that it's simply semantic word play. Efficiency is power out/power in. Sensitivity is power out/voltage in. Now in my mind, power and voltage are certainly not the same thing. Clearly, impedance is a factor.

I mean, if you have two speakers with equal sensitivities of 90dB/2.83V and one has a 4 ohm impedance and the other has an 8 ohm impedance, the latter speaker will have a higher efficiency, since the 2.83V will represent half the number of watts. So clearly you can't lump the two terms together. They don't correlate with one another.

What do you guys think ? I'm no EE, but I've been told that efficiency/sensitivity is just hair splitting. I don't agree but I would like to know from the more experienced guys here. Thanks.
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Old 06-15-10, 06:55 AM
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Anthony

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Re: Speaker efficiency vs sensitivity

Ah, I see the disconnect.

Sensitivity is usually spec'd for specific conditions, all of which must be considered. A good speaker or driver will post exactly how they came to that. For example: 86dB@1W/1m. That Watt, though was made by driving the speaker at a certain voltage decided by the impedance. A 4Ω speaker develops more power for the same voltage (P=V^2/R), so to keep the 1W constant, they have to lower the voltage.

So you are right, for 90dB@2.83Vinput, the 4Ω and 8Ω speakers are completely different. In this case the 4Ω speaker was actually tested at 2W and the 8Ω speaker at 1W, so the 4Ω speaker is 3dB less efficient.

So the terms are interchangeable, if you have all the test data. Sadly, many spec's lack this or handwave it by using voltage instead of wattage (hoping you don't realize that 86dB was at 2W of power on the 4Ω speaker and not 1W when comparing speakers/drivers).

Clear as mud?
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Old 06-15-10, 07:04 AM Thread Starter
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Ice10

Join Date: Apr 2007
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Re: Speaker efficiency vs sensitivity

Is it true that doubling the efficiency nets a 3 db sensitivity or is that just 'in a perfect world' ? Because the EE basically told me that with sensitivty and efficiency there is a direct relationship (without mentioning any other variables). Isn't that a blanket claim ?
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Old 06-15-10, 07:10 AM Thread Starter
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Ice10

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Re: Speaker efficiency vs sensitivity

Also, if sensitivity is power out/voltage in and efficiency is power out/power in, I don't see how the relationship could be interchangeable since voltage and power are different.

I mean, can you say that a speaker that is more efficient is more sensitive ? I'm sure you could, but on it's own it's just a blanket claim. Take that example I mentioned with the 4 ohm and 8 ohm speaker. Assuming identical efficiencies, the 8 ohm speaker would be -3 dB's less sensitive. So it's not a scalable metric.

As I understand it, when you add in several real world variables the differences between sensitivity and efficiency become more apparent I think. I'm not good with equations which is what the EE was flashing around, but he didn't seem to understand why there would be big differences between the two.
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Old 06-15-10, 07:13 AM
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Anthony

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Re: Speaker efficiency vs sensitivity

I think what he meant was.

Pout=Pin*efficiency

assume Pin is constant

Pout_new/ Pout_original = (Pin * eff_new)/(Pin * eff_orig) = eff_new/eff_orig

in dB terms this is 10 log (eff_new / eff_orig) = dB so in this case 3 = 10 log (eff_new/eff_orig). Solve for eff_new/eff_orig and we get 2. Put it all together and

Pout_new / Pout_original = 2

A lot of algebra there, but basically in a base 10 dB scale, 3dB represents a doubling or halving (depending on the sign) of whatever value you are measuring.
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Old 06-15-10, 07:19 AM
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Anthony

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Re: Speaker efficiency vs sensitivity

Quote:
Vaughan100 wrote: View Post
Also, if sensitivity is power out/voltage in and efficiency is power out/power in, I don't see how the relationship could be interchangeable since voltage and power are different.

I mean, can you say that a speaker that is more efficient is more sensitive ? I'm sure you could, but on it's own it's just a blanket claim. Take that example I mentioned with the 4 ohm and 8 ohm speaker. Assuming identical efficiencies, the 8 ohm speaker would be -3 dB's less sensitive. So it's not a scalable metric.

As I understand it, when you add in several real world variables the differences between sensitivity and efficiency become more apparent I think. I'm not good with equations which is what the EE was flashing around, but he didn't seem to understand why there would be big differences between the two.

I think the real issue is that you are using sensitivity to mean "voltage sensitivity", while others use it to mean power sensitivity. In your example, the 4 and 8 Ω speakers are not the same speaker, so it reasons that they would have different sensitivities even if the stated voltage sensitivity is the same (since the same voltage on 1 is double the power on the other).

I see your point, and I think we are pretty close in agreement: you have to have all the details to exchange one for the other. Otherwise, that might be a bad assumption. It is not a difference between them, since most drivers fall into the 4Ω or 8Ω category and the results are easy to standardize.
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Old 06-15-10, 07:54 AM Thread Starter
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Ice10

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Re: Speaker efficiency vs sensitivity

Yes, the 4 ohm and 8 ohm speaker are not the same, but only have different impedance. So clearly the impedance will change the efficiency or sensitivity.

Would I be correct in assuming that if one increased sensitivity between two speakers and the impedance curve was identical in both, that the efficiency would also increase ? Two identically, linear impedance curves.

But that's the thing. Speakers don't have identical impedance curves so then claiming that the higher the efficiency, the higher the sensitivity doesn't have any basis in truth in the real world because there are several variables that come into play.

I take it that the doubling of efficiency = 3dB increase in sensitivity is an equation that doesn't include any other real world variables. It's just a theoretical number.

I'm just trying to understand this because it's difficult for me to understand it. I thought the definitions for sensitivity and efficiency were not up for debate re voltage and power. Other EE's I've spoken with have told me sensitivity concerns itself with voltage...efficiency with power....but when you introduce impedance to the equation both terms don't scale in a linear fashion.

Sorry for the rant, I'm just talking to myself.
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Old 06-15-10, 07:58 AM Thread Starter
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Ice10

Join Date: Apr 2007
Posts: 335
Re: Speaker efficiency vs sensitivity

Let me post something from the EE to give you an idea :

"Yes, it is obviously clear (as I also stated) that sensitivity and efficiency are calculated in different ways; the one is after all given in dB and the other as a ratio! But both is dependant on the varying impedance - both are average values integrated over a frequency band, both vary in the same manner, and when talking about relative differences between loudspeakers, they indicate the same tendencies: doubling the efficiency gives a 3dB rise in sensitivity and vice versa. You mention mathematics; perhaps one could cut qualitative statements short by looking at the simple conversion factor between the two:

Sensitivity(dB) = 112 + 10*log(efficiency)

That is a direct relationship without other variables.
"

That's his post but he doesn't believe that there are big differences between both terms. I've been led to believe that both terms are not interchangeable and there are several sources that also claim that people erroneously make the mistake of confusing both for the same thing.
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Old 06-15-10, 11:29 AM
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fbov

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Re: Speaker efficiency vs sensitivity

When two quantities are related by single-valued mathematical relationships, they can be used interchangably.

There is a seconary issue in that speaker sensitivity is sometimes expressed at constant voltage, instead of constant power. That doesn't change the interchangeability, it just adds impedence to the relationship. More complex, but perhaps better for comparison in some cases.

Have fun,
Frank
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Old 06-15-10, 11:40 AM
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Josh

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Re: Speaker efficiency vs sensitivity

Right. Sensitivity is measured by voltage but only in so far as the voltage applied equates to 1w of power into the nominal load. Many people don't understand this and marketing/manufacturers will take advantage of that by posting sensitivities measured with 2w or even 3w.
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